Sources:

[1a]
Kicksecure
Mobile Phone Security
https://www.kicksecure.com/wiki/Mobile_Phone_Security

[2a]
madaidans insecurities
Whonix Dev
https://madaidans-insecurities.github.io/messengers.html
[2b]
https://eprint.iacr.org/2015/1177.pdf

Luke Smith
Don't Use Telegram. Don't Use Telegram. Don't Use Telegram. Don't Use Telegram. Don't Use Telegram.
https://invidious.jing.rocks/watch?v=qBTsUVrCDAQ

[3a]
Mental Outlaw
Telegram Hands Over User Data to German Police
https://invidious.jing.rocks/watch?v=t7HZrpJ4OyQ

[3b]
Mental Outlaw
Telegram Review
https://invidious.jing.rocks/watch?v=pZEH04nJ_hE

[3c]
Android Central
Telegram reportedly gives user data to German authorities
https://www.androidcentral.com/apps-software/telegram-reportedly-gives-user-data-to-german-authorities